1. Field of the Invention
The present invention relates to welding equipment and particularly to pipe resistance butt welding apparatus adapted for displacement within the pipes being weld-joined.
Most advantageously the invention can be used for welding large diameter pipes in laying oil- and gas pipelines.
One of the problems encountered in operating conventional in-pipe welding apparatus consists in that it is rather difficult to manipulate them when removing flash, or when the apparatus drive fails.
2. Description of the Prior Art
Many attempts have been made to provide apparatus which allow a welding process to be mechanized. For a long period of time, primary emphasis was given to mechanization and automation of displacement of the welding torches or electrodes along the edges of pipe ends being welded together. The apparatus designed for this purpose practically simulated the movements of the operator but failed to appreciably enhance the welding efficiency (see U.S. Pat. No. 3,277,567) because of the principle followed which consisted in that in order to weld the edges of the pipe ends together, the space therebetween had to be filled with the metal of the electrode being melted.
The method of resistance butt welding has permitted the welding efficiency to be appreciably enhanced. To realize this method, welding apparatus were developed which can be subdivided into three types:
(1) out-of-pipe welding apparatus where all the apparatus' devices are disposed outside of the pipes being welded together;
(2) in-pipe welding apparatus;
(3) combined welding apparatus.
The practice has shown that in laying trunk pipelines over an off-road terrain, preferable are in-pipe welding apparatus, because the inside surface of pipes is an ideal guide for moving such apparatus within the pipes being butt weld. This same factor favours a more accurate alignment of the pipes and enhances the efficiency of the butt welding process.
Known in the art is an in-pipe apparatus for pipe resistance butt welding (see USSR Inventor's Certificate No. 286,103), comprising a body with an elongated carrying member. The apparatus has two internal expanding clamps consisting of current-feeding shoes. One of said clamps is rigidly attached to the body, whereas the other is movably mounted on the elongated carrying member and connected with a drive intended to enable reciprocating motion of said other clamp. A trimming mechanism for removing flash is mounted on the internal expanding clamps. Also, the apparatus has centering supporting rollers and driving wheels. The centering supporting rollers are mounted on the body by means of spring-loaded levers so as to be constantly pressed against the internal surface of the pipes being welded. The driving wheels are mounted on the body of the apparatus with the aid of spring-loaded levers so as to be constantly pressed against the inside surface of the pipes being butt-welded. Thus, the driving wheels support one end of the apparatus. The driving wheels are connected with a rotation drive enabling rotation of said wheels.
The main disadvantage of the above-described welding apparatus consists in the following. If the rotation drive providing for the movement of the apparatus within the pipes becomes disabled, a great tractive force is required to withdraw the apparatus from the pipe. This derives from the fact that in the course of withdrawing the apparatus from the pipe the driving wheels induce rotation of the transmission and the rotation drive, and these latter create a high resistance to the wheels rolling. At the same time, it is essential that the driving wheels be in contact with the inside surface of the pipe to insure against the apparatus displacement under gravity at inclined sections of the pipeline. It will be understood that the constant contact between the wheels and the inside surface of the pipe is also necessary for maintaining and centering of one end of the apparatus.
The constant contact between the driving wheels and the inside surface of the pipe also presented a problem in a flash removal. In particular, the resistance created by the rotation drive and the transmissions had to be overcome by the drive enabling reciprocating motion of the clamps carrying the trimming mechanism.